Conventional multimode interferometers (MMI) couplers 1, illustrated in FIG. 1A, are fabricated from a single layer of waveguide material, e.g. silicon, including a width w_mmi and a thickness h_mmi, typically 220 nm thick. Single layer MMI couplers result in a high modal phase error, which prevents the formation of good images essential to the realization of a good MMI, characterized by a low insertion loss, small power imbalance, high common mode rejection ratio (CMRR), and small phase error. For an MMI coupler with a smaller number of input/output (IO) ports, such as 1×2 and 2×2, the high modal phase error of the MMI coupler is partly mitigated by the fact that only a very limited number of modes are supported in the MMI region, e.g. 2 to 4 modes. For MMI couplers that have a large number of IO ports, such as a 4×4, there is inevitably a larger number of supported modes in the multimode region, typically >10, as the width of the MMI core grows larger, and in order for these MMI couplers to form good images, the supported modes must have low modal phase error, which can't be achieved using a single layer of waveguide material, e.g. silicon. FIG. 2A illustrates a plot of phase error vs number of modes for a standard single layer MMI coupler 1 including different widths w_mmi, e.g. 4 um, 6 um and 8 um, and a single height h_mmi 0.21 um. The phase error grows exponentially with the number of supported modes and for each width w_mmi.
In order to realize an MMI coupler with a large number of input/output ports, such as a 4×4 MMI coupler, e.g. on a semiconductor (silicon) photonics platform, a dual layer MMI coupler 11, as illustrated in FIG. 1B, is usually comprised of a central core region 12, including a width w_mmi and a thickness h_hmmi, typically 220 nm thick, and a partially etched layer 13 including a height h_partial extending from each side of the central section 12. The partially etch layer 13 effectively reduces the index contrast between the core region 12 of the MMI coupler 11 and its side cladding 14 by increasing the index of the side cladding 14. The partially etched layer 13 provides the extra degree of freedom needed to control the effective indices of the supported modes, which can be used to minimize the modal phase error. In practice, dual layer MMI couplers 11 are very sensitive to the relative thickness of the full-height waveguide 12 versus the partially etch waveguide 13; therefore, when these MMI couplers are fabricated, thickness variation as small as 10 nm between the designed MMI coupler and the fabricated MMI coupler can result in unacceptable loss, balance, CMRR, and phase error. FIG. 2B illustrates a plot of modal phase error vs number of modes for a dual layer MMI coupler 11, for MMI's with different partial etch layer heights h_partial, but constant widths w_mmi or 7.7 um and heights h_mmi of 0.21 um. The dual layer MMI coupler 11 achieves smaller modal phase error than the single layer MMI coupler 1, but is sensitive to the thickness of the partial etch layer 13.
With reference to FIGS. 3A and 3B, both single and dual layer MMI couplers typically have single-layer, full-height access ports 16a-16d. The width of the access ports (w_access) is typically optimized for maximal coupling to the Nth or Mth order mode of the MMI core 12 of an N×M port MMI coupler 11, respectively. This optimal access port width, e.g. 1 μm to 2 μm on SOI 220 nm platform, is almost always wide enough to support multi-modes, e.g. w_access>0.5 μm. In theory, the light coming from the core 12 of the MMI coupler 11 forms an image that aligns with the access ports 16a-16d both in position and size, and the higher order access modes are not excited, as illustrated in FIG. 3A. In actuality, because of thickness variation in the layers, lithographic rounding, refractive index variations, wavelength dispersion, etc. the image that actually forms at the outputs 16a-16d of the MMI coupler 11 is distorted, as illustrated in FIG. 3B. The distortions are asymmetrical to the position and size of the access ports 16a-16d, which promotes coupling to high order modes supported in the access ports, as depicted in the bottom and top ports 16a and 16d. The relative phase shift between the high order modes and the fundamental mode of interest quickly accumulates in the access ports 16a-16d given the taper resulting in significant phase shift at the end of the taper where the width reaches the routing waveguide width. This results in the creating of highly undesirable ripples especially observable in phase. Note that each port 16a-16d can support the fundamental and higher order modes, such as the depicted second order mode, and that coupling to these modes can happen at each of the ports 16a-16d. 
An object of the present invention is to overcome the shortcomings of the prior art by providing an MMI coupler with a core comprise of strips of waveguide material and strips of cladding material.